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Abstract

Surface morphology and thermal stability of Cu-phthalocyanine (CuPc) films grown on
an epitaxially grown MgO(001) layer were investigated by using atomic force microscope
and X-ray diffractometer. The (002) textured β phase of CuPc films were prepared at
room temperature beyond the epitaxial MgO/Fe/MgO(001) buffer layer by the vacuum deposition
technique. The CuPc structure remained stable even after post-annealing at 350°C for
1 h under vacuum, which is an important advantage of device fabrication. In order
to improve the device performance, we investigated also current-voltage-luminescence
characteristics for the new top-emitting organic light-emitting diodes with different
thicknesses of CuPc layer.

Keywords:

Background

For charge injection into organic semiconductor (OSC) devices, oxide materials have
been widely utilized. In general, indium thin oxide (ITO) is used as a transparent
conducting electrode for optoelectronic devices which enable light to be emitted from
the bottom of the structure. The work functions and electronic properties of ITO surfaces
and, consequently, the interface electronic properties of contacts to OSC have become
important issues to improve the organic light-emitting diode (OLED) device performance
[1]. Different chemical and physical treatments have been investigated to control the
work function of an anode ITO electrode. In order to control efficiently the charge
injection and transport in OSC devices, a novel approach by introducing a thin oxide
layer between an anode and a hole-transport layer (HTL) in organic light-emitting
diodes (OLEDs) has gotten considerable attentions recently [2-4]. After the first report of the role of ITO as a charge generation layer (CGL) [5], several other studies followed to demonstrate the effect of CGL based on oxide films,
such as MoO3[6], V2O5[7], and WO3[1]. The improved device efficiency was attributed to the generation of holes which could
reduce the charge injection barriers at organic semiconductor interfaces upon application
of an electric field.

Considering the monolithic integration of an OLED on silicon or organic thin film
transistors as well as spin valves, a thin epitaxial MgO(001) layer as the CGL in
OLEDs might be interesting to investigate for magneto-optoelectronic applications.
The crystalline MgO(001) layer grown on Fe(001) is well known as a tunnel barrier
for the fully epitaxial Fe(001)/MgO(001)/Fe(001) tunnel junctions with a huge magnetoresistance
(MR) value at room temperature (RT) due to a band symmetry filtering effect [8,9]. Remarkable improvements have been recently achieved in the field of organic spin
valve devices [10,11]. However, according to our knowledge, up to now the investigation on the MgO-based
organic spin valves with a huge MR is not reported yet.

In order to understand the effect of an epitaxial MgO(001) layer on the charge generation
mechanism of OLED devices, a preliminary investigation to fabricate the hybrid multilayer
films with sharp amorphous or crystalline interfaces is required. However, the growth
of organic thin films beyond the metal or oxide contact was challenging; and a number
of problems had to be solved, especially to obtain smooth and ordered molecular monolayer
of the organic materials suitable for applications of organic spintronic devices [12]. Therefore, in this work, we investigated the growth of organic-inorganic hybrid
multilayer structures for Si(001) substrate-based OLED devices. In particular, we
focused on the growth of layer-structured HTL Cu-phthalocyanine (CuPc) and characterization
of its surface morphology after heat-treatment under vacuum, since CuPc is one of
the most popular OSC with high thermal and chemical stability suitable for thin film
organic devices [13]. Thus, it has been also widely used in optoelectronic devices [14].

Additionally, a new top-emitting OLED (TOLED) structure, which is formed on an opaque
Si(001) substrate and an epitaxial MgO(001)/Fe(001)/MgO(001) bottom electrode so that
light can emit from the thin Al top electrode, was investigated. Our TOLED design
includes a semi-transparent cathode Al, a stack of conventional organic electroluminescent
layers, and a thin CuPc film to enhance the hole injection into the electroluminescent
layers.

We expect that our new approach could open up the door toward the development of multifunctional
architectures for future device technology.

Methods

We used a ultra-high vacuum (UHV)-molecular beam epitaxy film evaporation system to
stack successively inorganic elements beyond chemically etched p-type Si(001) wafers.
The base pressure of the UHV-MBE chamber was lower than approximately 2 × 10−10 Torr. The epitaxial MgO(001)/Fe(001)/MgO(001) multilayers were formed at 250°C with
low deposition rate of 0.003 nm/s. During the deposition the pressure was kept lower
than 3 × 10−9 Torr.

In order to investigate the thin CuPc film growth mode and its structural properties,
20-nm-thick CuPc plane films were deposited at RT beyond Si(001)/8.0 nm MgO/15 nm
Fe/1.8 nm MgO by high vacuum thermal evaporation (base pressure at approximately 2
× 10−7 Torr). It should be noted that air-exposure on the MgO top surface is inevitable
during the sample transfer from the UHV-MBE chamber to the HV-thermal evaporator.

Figure 1 presents the schematic structure of multi layer TOLED. The 20-nm-thick Al cathode
was deposited on the OLED layer structure without incurring damage to the underlying
active organic emissive layer: CuPc as a hole-injection layer, N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) as a hole-transport layer and 8-tris-hydroxyquinoline aluminum (Alq3) as a light-emitting layer. During the deposition of the organic films, the pressure
was kept no more than 2 × 10−6 Torr, and the deposition rate was fixed at 0.03 nm/s. During deposition, the layer
thickness was obtained by a quartz thickness monitor and verified after deposition
by a step-profiler. The names of the TOLEDs with their structural information are
listed in Table 1.

Figure 1.Schematic structure of our OLED devices. MgO/Fe/MgO(001) structures were prepared beyond a chemically etched Si(001) wafer
by using UHV-MBE technique, then followed by deposition of CuPc/α-NPD/Alq3/Al multilayers at room temperature by using thermal evaporation in high vacuum.

For the Si(001)/MgO/Fe/MgO/CuPc hybrid multilayers before and after vacuum annealing
for 1 h in the temperature range from 100°C to 250°C, the structural characteristics
were examined by using X-ray powder diffraction technique (XRD) and atomic force microscope
(AFM). In order to analyze the microstructure of these hybrid multilayer interfaces,
cross-sectional samples for transmission electron microscope (TEM) were prepared using
conventional mechanical polishing and dimpling techniques. All the images were obtained
using a double aberration corrected JEOL FS2200 TEM (JEOL Ltd., Tokyo, Japan) with
atomic resolution. A new specimen preparation process with minimum damage onto the
organic layer was developed. We used mechanical thinning, followed by precision ion-beam
polisher system or (PIPS) with very short time to clean the surface. We have also
minimized the damage during the TEM observation by reducing the e-beam exposure time.
In addition we have confirmed the presence of the CuPc by energy dispersive X-ray
scan using the scanning TEM observation mode.

The current-voltage-luminance (I-V-L) characteristics were investigated by using a Keithley 236 source-measure unit and
a Keithley 2000 multimeter equipped with a photomultiplier tube through an ARC 275
monochromator (Keithley Instruments Inc., Cleveland, OH, USA). The external quantum
efficiency of the electroluminescence (EL), defined as the ratio of the emitted photons
to the injected electric charges, was calculated from the EL intensity measured by
using a calibrated Si photodiode placed at a normal angle to the device’s surface.

Results and discussion

The main goal of this study was to obtain an enhancement of the thermal durability
of the OLEDs. Particular attention was given to the engineering of charge injecting
contacts by introducing the epitaxial thin MgO(001) layer. Figure 2 shows an important enhancement of thermal stability of the 20-nm-thick CuPc film
grown the MgO(001) layer. Black color represents the result obtained before annealing,
while red, green and blue colors correspond to the results for the sample after 1
h vacuum annealing at 250°C, 300°C and 350°C, respectively. The dashed lines represent
CuPc, MgO and Fe peak positions from the powder diffraction file. After 1 h of vacuum
annealing in the temperature range from 150°C to 350°C, a strong diffraction peak,
corresponding to the (002) lattice plane of β-phase CuPc, persists at 2θ = 7.15° which is quite close to the position expected from the reference data. The
peak intensity increases as the annealing temperature increases up to 250°C, while
the intensity decreases after the annealing at 350°C. These XRD patterns indicate
that the Fe and MgO buffer layers are strongly (001) textured, only the (002) diffraction
peaks of bcc Fe and fcc MgO are observed. Consequently, the CuPc films deposited on
the epitaxial Fe/MgO(001) buffer layer are also predominantly (001) textured. Although
several earlier works reported the occurrence of a phase transition between α and
β phases by thermal treatment above 200°C [15,16], but such a phase transition was not observed in this work.

Figure 2.X-ray diffraction θ-2θ scans with Cu Kα radiation for the 20-nm-thick CuPc films before and after thermal
treatment.

Figure 3 shows the surface morphology of 20-nm-thick CuPc films (Figure 3a) before and after annealing at (Figure 3b) 150°C, and (Figure 3c) 250°C, respectively. Relatively smooth surfaces with root mean square (RMS) roughness
of less than 2 nm were observed after the annealing up to 250°C. However, the RMS
roughness became larger by annealing at the temperature higher than 250°C (Table 2). This is quite consistent with the strengthening of the [002] CuPc texture with
increasing post-annealing temperature found by the XRD. This can simply reveal the
enhancement of thermal stability and crystallinity of the CuPc layered structure due
to the MgO(001) underlayer effect. From the AFM surface analysis, rather homogeneous
roughness distributions for the vertical distance between the highest peak and the
lowest valley were observed in the range from −4 to 4 nm.

Table 2.RMS roughness for CuPc films after post-annealing for 1 h under vacuum

Here, we report a hybrid system consisting of a highly qualified interface between
the MgO/Fe/MgO(001) and the OSC CuPc. Figure 4 corresponds to the TEM image for the 20-nm-thick CuPc film grown at RT. The image
shows, from bottom to top, the Si substrate, the 8-nm-thick MgO buffer layer, and
the 10-nm-thick metallic Fe epilayer covered with the 1.8-nm-thick MgO. Note that
the MgO/Fe/MgO(001) multilayers are well-crystallized, but some layer roughness is
observable.

Figure 5 shows the current density-voltage (J-V) (Figure 5a) and the luminance-voltage (L-V) (Figure 5b) characteristics for the TOLED devices with 10-nm-thick CuPc film prepared beyond
the different anodes, such as Al (blue, S1), polycrystalline Fe (poly-Fe) (black,
S4), Fe(001) with (green, S2) and without MgO(001) (red, S3). For more details of
the TOLED structure, see Table 1.

When compared to the TOLED devices based on Al and poly-Fe and Fe(001) bottom electrodes,
the threshold voltage for the TOLED based on the Fe/MgO(001) electrode increases significantly.
More drastic increase in the driving voltage is also shown for that Fe/MgO(001)-based
TOLED. The large driving voltage could be attributed to the larger work function of
the MgO(001) layer (4.94 eV) [17] than that of Al (4.1 eV) [18] or poly-Fe (4.5 eV) [18]. Additionally, the effect of surface potential at the MgO(001)/CuPc interface could
not be negligible. Since it was reported that charge injection barriers at metal/organic
or oxide/organic interfaces affect the charge injection and recombination significantly
[19-21], we also investigated the IV (Figure 6a) and LV (Figure 6b) characteristics for the Fe/MgO(001)-based TOLED with different CuPc thicknesses:
15 nm (red, S5), 5 nm (green, S6) and 1 nm (black, S7) as shown in Figure 6. The structure information of S5, 6 and 7 TOLED are given in Table 1. EL spectra of TOLED with different thickness of CuPc are shown in Figure 3c: red for S5, green for S6, and black for S7. When the thickness decreases from 15
to 1 nm, no remarkable change appeared in the threshold and driving voltages, but
current and EL intensity increased obviously. A largely enhanced EL was observed in
the TOLED with 1-nm-thick CuPc layer. To improve the TOLED device performance, further
studies to optimize the device structure and fabrication conditions are required;
the EL efficiency is far from perfect. However, our results suggest a new possibility
to integrate spintronics with organic electronics: The use of the epitaxial thin MgO(001)
layer is proposed not only to improve the performance and the stability of OLED, but
also to inject the fully polarized spin current from the Fe/MgO(001) interface to
the OSC layers [9]. Indeed, the enhanced thermal stability of a few-nanometer-thick CuPc films could
result from the MgO(001) underlayer effect: The (002)-textured β phase of CuPc layer
persists even after the vacuum annealing at 350°C. Significant work function alteration
by inserting MgO(001) between ferromagnetic metal and the CuPc OSC layer could provide
a wide versatility of device functionality. For example, polarized light could be
generated by fully polarized spin injection through the Fe/MgO(001) interface. Thus,
for future work, it is worth to study the magnetic field effect in this OLED device.

Conclusions

In order to control the operation of an OLEDs, we investigated a new possibility of
the use of thin MgO(001) layer between the ferromagnetic metal Fe(001)-OSC CuPc interface
as a organic surface modifier. Remarkably enhanced thermal stability of CuPc films
with smooth surface morphology (RMS roughness ≤ approximately 2 nm) was obtained up
to the temperature of 350°C. In this work, the use of appropriate oxide layers could
represent a new interface engineering technique for improving reliability and functionality
in OSC devices. Based on the reliable Si(001)/MgO(001)/Fe(001)/MgO(001)/CuPc hybrid
stack, the new TOLED structure was investigated for future organic spintronic device
applications.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

YJB and THK carried out the overall experiment and data analysis. NJL did the fabrication
of OLEDs and the IVL experiments. HC and CL gave advice and helped for fabrication
of OLEDs. LF and AH performed the TEM experiments. All authors read and approved the
final manuscript.

Acknowledgements

This research is funded by the National Research Foundation of Korea (NRF) grants
funded by the Ministry of Education, Science and Technology (MEST) (no. 2008-0062239,
2011-0017209, and 2010-0006749). YJB thanks the NRF (no. 2011-0001809) for the financial
support.